This invention relates generally to photoelectric conversion devices, and more particularly to a photoelectric conversion device such as a solar cell in which its electrodes are formed by printing an electrode composition on a semiconductor substrate so that a photoelectric conversion device such as a solar cell having a high conversion efficiency can be easily manufactured at a low cost.
The screen printing technique has become more and more popular as an inexpensive and effective means for forming electrodes of a solar cell. According to this screen printing technique, a viscous slurry (which will be referred to hereinafter as a conductive paste) prepared by mixing a metal or metals in powder form with a powdery vitreous or glass material and dispersing the mixture in an organic solvent is printed by a stencil screen on a semiconductor substrate which is to be formed with the electrodes, and is then subjected to firing treatment at a suitable temperature to provide the desired electrodes. Such a screen printing technique is generally employed for the manufacture of a semiconductor device such as a thick-film IC device in which resistors, capacitors and other circuit elements are integrated to provide an integrated circuit configuration.
Application of this screen printing technique to the formation of the electrodes of the solar cell is advantageous over the conventional vacuum evaporation technique and plating technique in that the step of formation of the electrodes can be greatly improved so that the process for the manufacture of the solar cell can be greatly simplified and can also be automated for mass production.
The reduction in the manufacturing cost of solar cells is one of the most important subjects at present and is the critical or determinative factor for the extensive use of solar cells. It is acknowledged in this connection that the step of formation of the electrodes occupies a large proportion of the cost required for the manufacture of the solar cells, and improvement of this step is now strongly demanded. From this standpoint, the aforementioned screen printing technique is most suitable for the desired reduction in the solar cell manufacturing cost.
In the application of this screen printing technique to the formation of the electrodes of a solar cell comprising a photovoltaic semiconductor substrate of silicon, there are the important requirements that each of the electrodes formed by screen printing exhibit a low contact resistance between it and the semiconductor substrate and be firmly deposited on the semiconductor substrate and that the electrode material deposited on a diffused layer having a conductivity type different from or opposite to that of the semiconductor substrate should not penetrate through this diffused layer.
It has been reported that a conductive paste of silver (Ag) commonly used hitherto for the formation of electrodes of resistors, capacitors, etc. by screen printing and subsequent firing treatment is also used for the formation of the electrodes of the solar cell of the kind above described. It has also been proposed that a conductive paste of silver/palladium (Ag/Pd) or gold (Au) commonly used hitherto as the electrode material for resistors and capacitors can also be used for the above purpose. The conductive paste of Ag or Ag/Pd contains Ag in powder form or a mixture of Ag and Pd in powder form, a powdery low-melting glass composition containing glass of the lead oxide (PbO) system as its principal component, or powdery borosilicate-lead glass, an organic binder such as ethyl cellulose, and an organic solvent such as cellosolve added to the above composition to adjust the viscosity of the conductive paste which is applied in the form of a slurry during printing.
However, the researches and studies made by the inventors have proved that, when such a conductive paste is merely printed on a semiconductor substrate of a solar cell and then fired to provide the desired electrodes, a barrier tends to be formed between each of the electrodes and the semiconductor substrate resulting in difficulty of ensuring the desired satisfactory ohmic contact as well as the desired low contact resistance.
In order to form the electrodes on a photovoltaic semiconductor substrate of, for example, n-type silicon by the use of such a conductive paste thereby providing a satisfactorily usable solar cell, it has been required that the impurity concentration at the silicon substrate surfaces to be formed with the electrodes, that is, at both the major surface diffused or doped with a p-type impurity to form a pn junction and the other non-doped major surface, be higher than 10.sup.19 cm.sup.-3 so that a p.sup.+ -type layer and an n.sup.+ -type layer can be formed in such surfaces respectively. It has also been required that the conductive paste layers printed on the substrate surfaces be fired at a relatively high temperature higher than 800.degree. C.
Another problem arises due to the firing treatment at such a high firing temperature. When, for example, boron is diffused into one of the major surfaces of the n-type silicon substrate to form a photovoltaic p.sup.+ -type diffused layer defining a p.sup.+ /n junction within the substrate, the electrode formed on the photovoltaic p.sup.+ -type diffused layer which is to be irradiated with the solar radiation in use may penetrate through this p.sup.+ -type diffused layer, tending to destroy the p.sup.+ /n junction exhibiting the photovoltaic characteristic.
Thus, extreme difficulty has been encountered when such a prior art conductive paste of Ag or Ag/Pd is used to form the electrodes of the solar cell in satisfying the important requirements including the low contact resistance and no penetration of the electrode through the photovoltaic diffused layer. The prior art conductive paste of Au has had a problem similar to that encountered with the use of the prior art conductive pastes of Ag or Ag/Pd above described. In addition, in the case of the prior art conductive paste of Au, it has been more difficult to provide an inexpensive solar cell due to the fact that the cost of this conductive paste is about ten or more times higher than that of Ag.
A suitable amount of a powdery vitreous material acting as a binder for the powders of the metal or metals is generally added to each of these conductive pastes so that the electrodes having been subjected to the firing treatment can be firmly and stably bonded to the semiconductor substrate.
The composition of such a powdery vitreous material varies generally depending on the kind of the metal component or components and also on the firing temperature. In the case of a conductive paste of Ag which is usually fired at a medium firing temperature of about 550.degree. C., the binder contains powdery low-melting glass of PbO as its principal component. On the other hand, in the case of a conductive paste of Ag which is usually fired at a firing temperature higher than 800.degree. C., the binder contains powdery borosilicate-lead glass as its principal component. Thus, the powdery vitreous material in the prior art conductive pastes contains a large amount of lead glass although these conductive pastes have different compositions and are fired at different firing temperatures. The presence of this lead glass in the conductive paste obstructs deposition of the electrodes making satisfactory direct ohmic contact with the semiconductor substrate of, for example, silicon. Such an obstruction occurs because the lead glass component in the vitreous binder has the tendency of promoting oxidation of the semiconductor substrate of, for example, silicon, during the step of firing of the conductive paste, and the oxide film thus formed on the semiconductor substrate intervenes between each of the electrodes and the associated substrate surface, resulting in a high contact resistance. Thus, a firing temperature higher than 800.degree. C. has heretofore been required in order to break the oxide film causing the high contact resistance, thereby ensuring satisfactory ohmic contact of the metal component in the paste with the semiconductor substrate.
It will be understood from the above discussion that these conductive pastes used hitherto for forming the electrodes of thick-film resistors and capacitors have not been satisfactorily usable as the electrode material for photoelectric conversion devices such as solar cells. It has therefore been difficult to provide solar cells having a high conversion efficiency by the use of these prior art conductive pastes as the electrode material therefor.